Balun transformers are transformers which transform a balanced electrical state to an unbalanced electrical state. Although balun transformers can be configured and analyzed in various ways, in the present application, the balun transformer is envisioned as being of transmission line form. Typical examples of the fabrication and analysis of balun transformers in transmission line can be found in Proceedings of the IRE, "Some Broad-band Transformers" by C. L. Ruthroff, August, 1959, pp. 1337, et seq.
In the present state of the electronics art, especially in applications where compact and sub-compact electronic circuitry is essential, circuits are formed on well known printed circuit boards. In fact, it has been recognized that balun transformers can be fabricated of the actual stripline or conductor material of a printed circuit board substrate. This recognition eliminates the need for a separate balun transformer component, which can be of great advantage in many applications.
One stripline balun is shown in U.S. Pat. No. 3,571,722. As shown in FIG. 1 of said patent, a first stripline of generally "U" shape is formed on one side of a printed circuit board substrate. On the opposite side of the substrate is a patterned ground plane including an isolation gap of generally rectangular shape located generally within the confines of the "U". In the embodiment illustrated, the length of the legs of the "U" total approximately one-half wavelength. At the frequencies of about 975-1220 MHz, for instance, such dimensions would result in each side being about an inch in length, so that the entire assembly would be about two inches square. This size is undesirably large for many printed circuit board applications, especially where space is at a premium.
Another example of a microstrip balun is shown in U.S. Pat. No. 3,652,941. This patent shows the use of microstrip baluns to connect an RF input and a local oscillator input, both in unbalanced states, to respective input terminals of a quad diode race in a balanced state. Although this patent teaches microstrip baluns, the resulting structure is unlike that of applicants' in that it requires an unduly large linear dimension to achieve its effective purpose.